1. Field of the Invention
The present invention relates to a light reflecting mask, an exposure apparatus, and a measuring method used for lithography using extreme ultraviolet (EUV) light.
2. Description of the Related Art
In lithography using EUV light having a wavelength of about 13.5 nm, a light reflecting mask capable of transferring a pattern by reflecting exposure light is used. The light reflecting mask is formed by, e.g., sequentially stacking, on a glass substrate, a reflecting layer which reflects light, a cap layer made of silicon (Si) or the like, and an absorbing layer which absorbs light. A pattern is formed by partially etching the absorbing layer.
The reflectance of the light reflecting mask having such a structure decreases along with the repeat of exposure because of the influence of oxidation or corrosion of the exposed cap layer surface or substances sticking to it. This degrades the image contrast when transferring the pattern onto a wafer.
A reference (Jpn. Pat. Appln. KOKAI Publication No. 2004-61177) discloses a technique of measuring degradation in an optical element such as a mirror arranged in an exposure apparatus. However, regarding degradation in a light reflecting mask, it is difficult to measure the characteristics and change because the pattern formed on the mask is not uniform in general.